Conventional electrical systems that utilise electrical switching, wiring and related interconnections may degrade with time, temperature, moisture, vibration, or other environmental conditions. Such degradation may result in electrical arcing events that increase an operating temperature of the electrical system and, in extreme conditions, lead to fire.
Arcing faults mainly occur in one of two ways, series arcing faults or parallel arcing faults. A series arc can occur due to an unintentional break in an electrical system conduction path for its load. A series arc is load limited because the arc current cannot be greater than the current that the conduction path provides to its load. Conduction path current that experiences series arcing has a lower root mean square (rms) value than current without such series arcing due to extinction and re-ignition.
Parallel arcing faults either occur because of an electrical system conduction path that experiences a short circuit or a ground fault. The only limit for the current flow of a parallel arc is due to the electrical system impedance and the impedance of the fault itself. The rms current value for parallel arc faults may be considerably less than that of a solid fault so that ordinary circuit breaker systems may not detect such faults before a they create a fire.
Such electrical arcing, whether series or parallel, may be inherently low impedance and occur infrequently. Such infrequent occurrence may prevent a thermal circuit breaker from reaching its predetermined temperature trip level or a solid-state power controller (SSPC) from reaching its predetermined energy trip rating. As a result, conventional thermal circuit breakers and SSPCs typically do not trip when such electrical arcing occurs, even though it is desirable to detect electrical arc events for purposes such as identifying an electrical problem and preventing the problem from causing serious damage or hazards.
Electrical arc detection hardware is available for electrical arc detection. Such hardware may couple to a thermal circuit breaker to detect an AC electrical arc and initiate a trip of the breaker. Likewise, such hardware may couple to a SSPC to detect an AC electric arc and initiate a trip of the SSPC. However, such electrical arc detection increases the size, weight and cost of the electrical system.
U.S. Ser. No. 11/297,862 to Oldenburg et al., owned by the assignee of this application and hereby incorporated into this application by reference, describes one effective solution for detecting electrical arc faults of both the series and parallel type that is suitable for incorporating into an SSPC with no additional hardware.
Such electrical arc fault detection solutions typically compare a sample of the current with a previous sample to determine if arcing differences and characteristics exist. For maximum response, the comparison is usually on a half AC cycle basis. Certain AC loads, typically those that comprise electrical rectification, switching, and other non-linear functions may cause a direct current (DC) offset in the load current. A small DC offset in the AC load current is insignificant with respect to the overall load at the power source level, but it may be significant at a lower distribution level.
The DC offset results in making the AC load current appear asymmetrical. Arc fault detection solutions typically rely on sampled current to determine if arcing is occurring. Samples of the asymmetrical current can, under the right circumstances, make such arc fault detection solutions detect and nuisance trip on these current samples.